Method and device for controlling an exhaust gas aftertreatment system

ABSTRACT

A method and a device are described for controlling an exhaust gas aftertreatment system for an internal combustion engine. A state variable is ascertained, which characterizes the state of the exhaust gas aftertreatment system. The temperature of the exhaust gas aftertreatment system is controlled as a function of the state of the exhaust gas aftertreatment system and/or of the internal combustion engine.

FIELD OF THE INVENTION

The present invention relates to a method and a device for controllingan exhaust gas aftertreatment system, in particular in a motor vehicle.

BACKGROUND INFORMATION

A conventional method and a device for controlling an exhaust gasaftertreatment system of an internal combustion engine have beendescribed, for example, in German Patent No. 199 06 287. In thatreference, the exhaust gas aftertreatment system includes at least oneparticulate filter, which is used particularly in direct-injectioninternal combustion engines. In the method described, a state variableis recorded, which characterizes the state of the exhaust gasaftertreatment system, such as the loading state of the particulatefilter. If this state variable, i.e. the loading of the particulatefilter, exceeds certain values, the device initiates a special operatingstate in which the particle filter is regenerated by suitable measures.It is also provided that fuel reaches the exhaust gas tract, which isoxidized in an oxidizing catalytic converter, in order to raise theexhaust gas temperature.

For the initiation and/or for carrying out the regeneration of theparticulate filter, additional fuel is required, which is either meteredin using an additional metering device in the exhaust gas tract, or withthe aid of the usual control elements for fuel injection. Thedisadvantage here is that the regeneration increases the fuelconsumption. Moreover, it is possible that, because of the regeneration,the exhaust gas temperature in the particulate filter increases toimpermissibly high values.

SUMMARY OF THE INVENTION

Because the temperature in the exhaust gas aftertreatment system,especially in the particulate filter, is controlled or regulated by avalue as a function of the state of the exhaust gas aftertreatmentsystem and of the state of the internal combustion engine, the increasedfuel consumption in the special operating state may be clearly reduced.Furthermore, the temperatures required for the special operating statemay safely be maintained. Temperature deviations to small or largevalues do not occur.

For this purpose, a control of the temperature of the exhaust gasaftertreatment system is carried out, especially of the temperature ofthe particulate filter, as a function of the state of the exhaust gasaftertreatment system and of the internal combustion engine. In thiscontext, in one embodiment, no feedback of the actual temperatureupstream of the particulate filter takes place, but rather thetemperature upstream of the particulate filter is ascertained usingother criteria, and on this basis it is determined whether theregeneration is to be ended. In particular, the temperature upstream ofthe oxidizing catalytic converter, which corresponds to the exhaust gastemperature of the internal combustion engine, is taken intoconsideration. This quantity may be both measured, and determined, fromother operating variables, such as the load and the rotational speed.

One specific embodiment is especially advantageous in which the controlsystem is configured in such a way that the temperature is measuredupstream of the particulate filter and compared to the setpoint value,and, starting from this comparison, the additional fuel quantity isdetermined.

It is advantageous if the special operating state is divided into atleast two phases. In a first phase, the amount of uncombusted fuel inthe exhaust gas may be increased over the course of time. In a secondphase, the amount of uncombusted fuel in the exhaust gas takes on aconstant value upstream of the oxidizing catalytic converter.

By this procedure it may be achieved that the temperature risesaccording to a desired function, that is, neither too fast nor tooslowly. The temperature may thus assume a constant value, or the controlvariables are adjusted in such a way that the temperature remainsconstant upstream of the particulate filter even at a variable operatingstate of the internal combustion engine. At an increase that is tooslow, the special operating state lasts too long. At too rapid anincrease, the particulate filter may be damaged, and uncombusted fuelmay reach the environment,

Because the duration of the first and/or second phase is predefined, theadditional amount of fuel may be adjusted to the current operatingstate.

Because the second phase ends when regeneration begins,the regenerationmay be speeded up, and, in addition, the consumption of fuel may befurther minimized.

One further refinement is particularly advantageous in which, in a thirdphase, the amount of uncombusted fuel is set intermittently to theconstant value, such as to the value of the second phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the device according to an embodiment ofthe present invention.

FIG. 2 shows a flow chart of the method according to an embodiment ofthe present invention.

FIG. 3 is a graph that illustrates the variation with time of theadditional amount of fuel.

DETAILED DESCRIPTION

FIG. 1 shows elements of an exhaust gas aftertreatment system of aninternal combustion engine according to an embodiment of the presentinvention. The internal combustion engine is denoted by 100. It issupplied with fresh air through a fresh-air pipe 105. The exhaust gasesof internal combustion engine 100 exit into the environment through anexhaust pipe 110. An exhaust gas aftertreatment system 115 is situatedin the exhaust gas line which, in the specific embodiment illustrated,includes a catalytic converter 115a and a particulate filter 115 b.Moreover, it is possible to provide several catalytic converters fordifferent pollutants, or combinations of at least one catalyticconverter and one particulate filter.

Also provided is a control unit 170 which includes at least one enginecontrol unit 175 and an exhaust gas aftertreatment control unit 172.Engine control unit 175 applies control signals to a fuel meteringsystem 180. Exhaust gas aftertreatment control unit 172 applies controlsignals to engine control unit 175 and, in one embodiment, to a controlelement 182 which is arranged in the exhaust pipe upstream of theexhaust-gas treatment system or in the exhaust-gas treatment systemitself.

Moreover, various sensors are provided which feed signals to the exhaustgas aftertreatment control unit and to the engine control unit. Thus,provision is made for at least one first sensor 194 which deliverssignals characterizing the state of the air which is fed to the internalcombustion engine. A second sensor 177 delivers signals characterizingthe state of fuel metering system 180. At least one third sensor 191delivers signals characterizing the state of the exhaust gas upstream ofthe exhaust gas aftertreatment system. At least one fourth sensor 193delivers signals characterizing the state of exhaust gas aftertreatmentsystem 115. Moreover, at least one sensor 192 delivers signalscharacterizing the state of the exhaust gases downstream of the exhaustgas aftertreatment system. These sensors may measure temperature valuesand/or pressure values. Moreover, sensors may also be used which detectthe chemical composition of the exhaust gas and/or of the fresh air.They may include, for example, lambda sensors, NOX sensors or HCsensors.

The output signals of first sensor 194, of third sensor 191, of fourthsensor 193 and of fifth sensor 192 may be applied to exhaust gasaftertreatment control unit 172. The output signals of second sensor 177may be applied to engine control unit 175. It is also possible toprovide further sensors (not shown) which detect a signal with respectto the driver's command, further ambient conditions, or engine operatingstates.

The engine control unit and the exhaust gas aftertreatment control unitmay form one structural unit. However, they may also be implemented astwo spatially separated control units.

In the following, the procedure of the present invention is describedusing as an example a particulate filter which is used particularly fordirect-injection internal combustion engines. However, the procedureaccording to the invention is not limited to this use; it may also beused for other internal combustion engines having an exhaust gasaftertreatment system. It can be used, in particular, in the case ofexhaust gas aftertreatment systems featuring a combination of acatalytic converter and a particulate filter. Furthermore, it may beused in systems which are furnished with only one or more catalyticconverters and/or one or more storage elements for gaseous exhaust gascomponents.

Based on the existing sensor signals, engine control 175 calculates thecontrol signals for sending to fuel metering system 180. This thenmeters in the appropriate fuel quantity to internal combustion engine100. During combustion, particulates can develop in the exhaust gas.They are trapped by the particulate filter in exhaust gas aftertreatmentsystem 115. In the course of operation, corresponding amounts ofparticulates accumulate in particulate filter 115. This impairs thefunctioning of the particulate filter and/or of the internal combustionengine. Therefore, provision is made for a regeneration process to beinitiated at certain intervals or when the particulate filter hasreached a certain loading condition. This regeneration is referred toherein as a “special operation”.

The loading state is detected, for example, on the basis of varioussensor signals. Thus, first of all, it is possible to evaluate thedifferential pressure between the input and the output of particulatefilter 115. Secondly, it is possible to ascertain the loading state onthe basis of different temperature and/or different pressure values. Inaddition, it is possible to utilize further variables to calculate orsimulate the loading condition. A suitable procedure is known, forexample, from German Patent No. 199 06 287.

When the exhaust gas aftertreatment control unit detects the particulatefilter to have reached a certain loading state, then the regeneration isinitialized. Various possibilities are available for regenerating theparticulate filter. Thus, first of all, provision may be made forcertain substances to be fed to the exhaust gas via control element 182,which then cause a corresponding reaction in exhaust gas aftertreatmentsystem 115. These additionally metered substances cause, inter alia, anincrease in temperature and/or an oxidation of the particulates in theparticulate filter. Thus, for example, provision can be made for fueland/or an oxidizing agent to be supplied via control element 182.

In one embodiment, provision can be made that a corresponding signal betransmitted to engine control unit 175 and that the engine control unitcarries out a so-called postinjection, such as a late postinjection. Thepostinjection makes it possible to selectively introduce hydrocarbonsinto the exhaust gas which contribute to the regeneration of the exhaustgas aftertreatment system 115 via an increase in temperature.

Usually, provision is made to determine the loading state on the basisof different variables. By comparison to a threshold value, thedifferent conditions are detected and the regeneration is initiated as afunction of the detected loading state.

In the specific embodiment shown, exhaust gas aftertreatment system 115includes an oxidizing catalytic converter 115 a as well as apost-connected particulate filter 115 b. The temperature (TV) upstreamof the catalytic converter may be recorded using sensor 191. Temperature(TN) downstream of the catalytic converter, which corresponds to thetemperature upstream of the particulate filter, is recorded using sensor193. In addition, a sensor 192 is provided which ascertains thedifferential pressure between the input and the output of particulatefilter 115 b. Furthermore, a device 182 is provided which introducesfuel into the exhaust gas tract, especially into exhaust gas pipe 110upstream of the oxidizing catalytic converter. As an alternative, it mayalso be provided that fuel gets to the exhaust gas tract via thecombustion chamber by suitable control of control element 180. It isimportant that uncombusted fuel should reach the oxidizing catalyticconverter. In this context, incompletely combusted fuel, which can beconverted in the oxidizing catalytic converter, is also considereduncombusted fuel.

The various variables with respect to the temperature and the pressuredifference may be recorded using the sensors shown, or calculated and/orsimulated by control unit 170, starting from other measured valuesand/or control signals which are present in control unit 170.

The fuel quantity introduced into the exhaust gas tract reacts in theoxidizing catalytic converter and may be burned there in a flame-freecombustion. This leads to an increase in temperature downstream ofoxidizing catalytic converter 115 a. According to the present invention,a fuel quantity is metered in such a way that the temperature increasesto a value that is required for the regeneration of the particulatefilter. The regeneration of the particulate filter takes place attemperatures above a certain value, which typically lies in the range of300° C. and 650° C., depending to an extent on the particular design ofthe exhaust gas aftertreatment system and the nature of the particulatelayer in the filter.

At exhaust gas temperatures that are too high,the particulate filter maybe damaged by overheating. This is a particular problem if a largequantity of particulates in the filter is converted, leading to anadditional temperature increase. If, on the other hand, the exhaust gastemperature is too low and/or the gas volume flow in the exhaust gas istoo high, a part of the fuel is reacted in the oxidizing catalyticconverter and the rest gets out uncombusted into the environment.

The procedural method according to the present invention is describedbelow with the aid of the flow diagram of FIG. 2. In a first step 200,the loading state of the particulate filter is determined, i.e. a statevariable P, which characterizes the state of the exhaust gasaftertreatment system is ascertained. This state variable characterizesthe soot mass that has collected in particulate filter 115 b. Thedetermination of state variable P can be carried out in different ways.For example, it may be provided that the state variable be simulated,starting from various operating parameters of the internal combustionengine. Thus, for example, the state variable may be integrated overtime, starting from the fuel quantity injected, the rotational speed andfurther variables. For this purpose, the generated soot mass is read outfrom a characteristics map for each operating point, and is summed up.In another specific embodiment, the pressure loss over the particulatefilter is measured. For this purpose, a differential pressure sensor isemployed which yields a pressure quantity that corresponds to thepressure difference between the input and the output of the particulatefilter.

Subsequent interrogation 210 checks whether this state variable P isgreater than a threshold value PSW. A regeneration of the particulatefilter is necessary in this case. If this is not the case, then step 200follows again.

If a regeneration is necessary, there follows interrogation 210.Interrogation 210 checks whether an operating point is at hand which isfavorable for a regeneration. Favorable operating points are operatingpoints where the exhaust gas temperature does not take on values thatare too low, and the gas flow does not take on values that are too high.Such operating points may occur when the injected fuel quantity takes onhigh values, and thus, in the simplest specific embodiment it is checkedwhether fuel quantity QK, which is injected, is greater than a thresholdvalue QKSW. It may further be provided to check whether the quotient ofthe injected fuel quantity QK and the gas flow is greater than athreshold value. If this is not the case, then step 200 is repeated.

If the operating point is favorable, step 230 is carried out, in whichthe regeneration is initiated. Alternative to interrogations 210 and220, other procedures may also be used to decide whether a regenerationis to be carried out. It may especially be provided that the twointerrogations 210 and 220 may be exchanged in their time sequence. Itmay additionally be provided that further interrogations are provided,so that, for example, in the case of a partial loading, a regenerationis carried out only when the operating point is favorable. If the statevariable reaches a value which lies substantially above the thresholdvalue for the state variable, initiation of regeneration takes place,independent of the operating point.

In step 230, temperature TV upstream of the oxidizing catalyticconverter is ascertained. For this purpose, temperature TV is stored ina characteristics map, as a function of various operating parameters ofthe internal combustion engine. It is especially advantageous if, inthis case, rotational speed N and the engine load are taken intoconsideration. As the load quantity, the fuel quantity to be injected isused. It is particularly advantageous if this quantity, read out fromthe characteristics map, is corrected for the compensation of theinfluences of outside temperature and travel wind cooling, usingcorrection factors. In this context, the correction factors arespecified as a function of the outside temperature and/or the travelspeed of the vehicle. It is of advantage that all these variables areavailable in control unit 175, and thus no additional sensors arerequired.

In subsequent step 240, the dosing pattern is fixed. The dosing patternis defined by the variation with time of the additionally metered-infuel quantity QZ. It may be provided that, during dosing, at least twophases are provided. In a first phase, additional quantity QZ rises fromthe value 0 to a constant value QKZ. It may be provided that theincrease follows a parabola. Alternatively, it may also be provided thata linear increase be provided. Constant value QKZ, to which theadditional fuel quantity is increased, may be determined starting fromtemperature TV upstream of the catalytic converter, rotational speed Nand the load of the internal combustion engine. Thus, in relation tothese magnitudes, i.e. the temperature upstream of the catalyticconverter, the desired temperature downstream of the catalytic converterand further operating characteristic values such as the rotational speedand the load, additional fuel quantity QZ is determined. Thisdetermination may take place with the aid of a characteristics map. Itis advantageous if the initial increase of the increase, and thus theduration of the increase, may be predefined as a function of thetemperature upstream of the catalytic converter.

In a further embodiment of the present invention, the dosing quantityafter expiration of the first phase is set so that the exhaust gastemperature upstream of the particulate filter remains constant even ifthe operating state of the internal combustion engine changes.

In step 250, the additional fuel quantity is then metered in with thepredefined dosing pattern. Additional fuel quantity QZ, on the one hand,may be supplied directly to the exhaust gas tract, and alternatively itmay also be provided that the fuel quantity be metered in with the aidof the control element used for the fuel metering.

In step 260, temperature TV upstream of the catalytic converter isascertained continuously. Here, a characteristics map or a sensor aremay also be used. If the temperature changes upstream of the catalyticconverter, the additional fuel quantity QZ to be injected isappropriately recalculated and corrected.

Subsequent interrogation 270 checks whether the retention time hasexpired, i.e., the interrogation checks whether additional fuel is beingsupplied sufficiently long. For the implementation of thisinterrogation, several specific embodiments are available.

In a first simple implementation, it is provided that the regenerationbe ended after a specified time. In this context, a fixed specifiedduration or a duration may be selected which is specified as a functionof the operating state of the internal combustion engine. Thus, inaddition, in step 240 a time counter is set to 0, and in interrogation270 it is checked whether the time counter has exceed a specified value.

In a further embodiment, it is provided that the metering in of theadditional fuel be ended or interrupted when the regeneration of theparticulate filter has begun. For this, the regeneration initiation isdetected in the particulate filter. This may be done, for example, byevaluating temperature TN upstream of the particulate filter and thetemperature downstream of the particulate filter. If the temperaturedownstream of the particulate filter is greater than the temperatureupstream of the particulate filter, one may assume an incipientregeneration, since this leads to a temperature increase. If thetemperature downstream of the filter runs through a maximum shortlyafter the beginning of the retention time, soot burning has set in.Therefore, according to the present invention, it is checked whether thetemperature downstream of the filter is greater than upstream of thefilter, and, depending on this interrogation, it is decided that theretention time has expired. For the evaluation of the temperature, acorrection may be provided which takes into consideration the heatrelease of the particulate filter to the surroundings, especially theheat radiation.

Instead of the temperature sensor, other sensors may also be used, suchas a differential pressure sensor, which measures the pressuredifference upstream and downstream of the particulate filter, or asensor which records the exhaust gas composition upstream and downstreamof the particulate filter. For this, a so-called lambda sensor isparticularly suitable, which records the oxygen concentration in theexhaust gas. If the oxygen concentration downstream of the particulatefilter is less than that upstream of the particulate filter, a beginningregeneration is determined to be taking place.

A disadvantage of this procedure is that, during the metering in offuel, the conversion of NO, generated by the engine, to NO₂ issuppressed. When the metering in of additional fuel is eliminated orinterrupted, NO₂ is formed again in the oxidizing catalytic converter,which reacts with the particles in the particulate filter and leads toan additional particle breakdown.

It is advantageous if, after shutting off the additional fuel metering,it is periodically switched on and off again. By doing this, a decreasein the temperature during regeneration may be prevented.

In the specific embodiment as shown in FIG. 2, a control of thetemperature of the exhaust gas aftertreatment system and of theparticulate filter is carried out as a function of the state of theexhaust gas aftertreatment system, of the internal combustion engine,and of the particulate layer. In this context, no feedback of the actualtemperature takes place upstream or downstream of the particulatefilter, but rather, it is decided only on the basis of other criteriawhether regeneration is to be stopped. In particular, the temperatureupstream of the oxidizing catalytic converter, which corresponds to theexhaust gas temperature of the internal combustion engine, is taken intoconsideration. This quantity may be both measured, and advantageouslydetermined from other operating variables, such as the load and therotational speed.

One specific embodiment is especially advantageous in which the controlsystem configured in such a way that the temperature is measuredupstream of the particulate filter and compared to the setpoint value,and, starting from this comparison, the additional fuel quantity isdetermined.

In FIG. 3, the variation with time of the additional quantity QZ, whichis metered in for the regeneration, is shown in exemplary form. In afirst phase, between points in time t1 and t2, the additional quantityincreases from zero to a constant value QKZ. Up to point t2, theconstant quantity QKZ is metered in when the operating point isconstant. When the operating point changes, the dosing quantity may beadjusted so that the exhaust gas temperature upstream of the particulatefilter remains constant. At point t2, the retention time has expired,and the additional quantity is reduced to zero.

In one especially advantageous embodiment, which is shown by dottedlines, the additional quantity is briefly set to a constant value atpoint t3.

1. A method for controlling an exhaust gas aftertreatment system of aninternal combustion engine, comprising: determining at least one firststate variable characterizing a state of the exhaust gas aftertreatmentsystem; initiating a special operating state for regenerating theexhaust gas aftertreatment system as a function of the state variable;and controlling a temperature of the exhaust gas aftertreatment systemas a function of at least one of the state of the exhaust gasaftertreatment system and the state of the internal combustion engine bysupplying uncombusted fuel to the exhaust gas, wherein, in a firstphase, a quantity of uncombusted fuel in the exhaust gas increases overtime, and, in a second phase, a quantity of uncombusted fuel in theexhaust gas takes on a constant value.
 2. The method of claim 1, whereina duration of at least one of the first phase and the second phase isspecified.
 3. The method of claim 1, further comprising: ending thesecond phase when regeneration of the exhaust gas aftertreatment systembegins.
 4. The method of claim 1, wherein, in a third phase, a quantityof uncombusted fuel is intermittently set to a constant value.
 5. Themethod of claim 1, wherein the constant value is specified as a functionof at least one of a rotational speed, a fuel quantity to be injectedand a temperature upstream of the exhaust gas aftertreatment system. 6.The method of claim 1, further comprising: detecting a beginning ofregeneration based on at least one of a temperature and an exhaust gascomposition both upstream and downstream of the exhaust gasaftertreatment system.
 7. A device for controlling an exhaust gasaftertreatment system of an internal combustion engine, comprising: acontrol unit for determining at least one state variable thatcharacterizes a state of the exhaust gas aftertreatment system, andinitiating a special operating state for regenerating the exhaust gasaftertreatment system as a function of the state variable; anarrangement for controlling a temperature of the exhaust gasaftertreatment system as a function of at least one of the state of theexhaust gas aftertreatment system and the state of the internalcombustion engine; and an arrangement for supplying uncombusted fuel tothe exhaust gas, wherein, in a first phase, a quantity of uncombustedfuel in the exhaust gas increases over time, and, in a second phase, aquantity of uncombusted fuel in the exhaust gas takes on a constantvalue.